In Earth Not a Globe the sun is a projection upon the atmosphere/atmoplane.
An illustration of the projection from the above work:
If the sun is a projection upon the atmosphere, might that provide some insight to how the sun sets?
1) I cannot follow Mr. R. in his claim, that the sun would appear bigger, if you insert a projection plane between the sun and the observer. The projection on the plane is bigger, yes, but distorted to an ellipse. The angled view of the observer will compensate this, and the observer will see the same image as without that additional projection plane.
2) If there is a projection plane, why is it invisible and only the sun gets "projected"? There should be some haze or similar to be seen not only for the disk of the sun. But in clear nights we see the twinkle of the stars and not a foggy image. With best viewing conditions, we see a crisp clear disk of the sun, until it intersects with the horizon.
What happens to the view in the far distance? The atmosphere eventually builds up to a point where you cannot see past. One cannot see for infinity. The atmosphere is comprised of opaque atoms and molecules, and is not perfectly transparent.
In the far distance on a flat earth, yes the haze of the atmosphere will accumulate more and more, until atmosphere is opaque. On a flat earth the line of sight would always go through the lower parts of the atmosphere, were the atmosphere is denser.
But on a globe earth, the viewing line to the sun, would soon go through the higher parts of the atmosphere, as the lower parts of atmosphere are curved down, away from the viewing line, following earth curvature. And the higher parts of atmosphere by far are less dense or opaque than the lower parts.